GitHub Importer - Migrate issue events
## Problem to solve
When migrating projects from GitHub to GitLab, the issues are imported including any comments. However, any history/events are not migrated over. This means that, after the migration, the users would not be able to see who added/removed labels, milestones, assignees, etc.
For some users this is a dealbreaker that would prevent them from migrating to GitLab.
## Proposed solution
During project import from GitHub, for each imported issue, check if any of the identified events exist and import them into GitLab.
## Events list
| GitHub | ➡️ | GitLab |
| ------ | - | ------ |
|  | ➡️ |  |
|  | ➡️ |  |
|  | ➡️ |  |
|  | ➡️ |  |
|  | ➡️ |  |
|  | ➡️ |  |
|  | ➡️ |  |
|  | ➡️ |  |
|  | ➡️ |  |
|  | ➡️ |  |
## Iterations
This feature will be delivered in iterations, as each event can be delivered as a separate change. That said, the following iteration plan groups related events to avoid user confusion.
1. :footprints: GitHub Importer - Migrate issue closed/reopened events - https://gitlab.com/gitlab-org/gitlab/-/issues/354234
1. :footprints: GitHub Importer - Migrate issue un/labeled events - https://gitlab.com/gitlab-org/gitlab/-/issues/354235
1. :footprints: GitHub Importer - Migrate issue de/milestoned events - https://gitlab.com/gitlab-org/gitlab/-/issues/354236
1. :footprints: GitHub Importer - Migrate issue cross-referenced events - https://gitlab.com/gitlab-org/gitlab/-/issues/354238
1. :footprints: GitHub Importer - Migrate issue un/assigned events - https://gitlab.com/gitlab-org/gitlab/-/issues/354237
1. :footprints: GitHub Importer - Migrate issue renamed events - https://gitlab.com/gitlab-org/gitlab/-/issues/354239
## Technical details
Per [GitHub API documentation](https://docs.github.com/en/rest/reference/issues#list-issue-events):
* we can use **`GET` /repos/{owner}/{repo}/issues/{issue_number}/events** to get a list of events for an issue
## Documentation
* [ ] :book: User doc: https://docs.gitlab.com/ee/user/project/import/github.html
* [ ] :book: Developer doc: https://docs.gitlab.com/ee/development/github_importer.html
epic